Heat exchanger, and method for transferring heat

ABSTRACT

A heat exchanger is provided to efficiently transfer heat between air and a flow of refrigerant in a reversing air-sourced heat pump system. When the system is operating in heat pump mode, a flow of air is directed through the heat exchanger and is heated by the refrigerant. A portion of the flow of air is used to de-superheat the refrigerant in a first section of the heat exchanger, and is prevented from re-heating the sub-cooled refrigerant in another section of the heat exchanger after the remaining air has been heated by the refrigerant. The same heat exchanger can be used to cool a flow of air using expanded refrigerant when the system is operating in an air conditioning (cooling) mode.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation-In-Part of International PatentApplication No. PCT/US2013/023657, filed Jan. 29, 2013, which claimspriority to U.S. Provisional Application No. 61/649,046, filed May 18,2012, the entire contents of both of which are hereby incorporated byreference herein.

BACKGROUND

The present application relates generally to heat exchangers and methodsfor transferring heat between fluids, and more specifically, relates toheat exchangers and heat transfer in refrigerant systems.

Vapor compression systems are commonly used for refrigeration and/or airconditioning and/or heating, among other uses. In a typical vaporcompression system, a refrigerant, sometimes referred to as a workingfluid, is circulated through a continuous thermodynamic cycle in orderto transfer heat energy to or from a temperature and/or humiditycontrolled environment and from or to an uncontrolled ambientenvironment. While such vapor compression systems can vary in theirimplementation, they most often include at least one heat exchangeroperating as an evaporator, and at least one other heat exchangeroperating as a condenser.

In systems of the aforementioned kind, a refrigerant typically enters anevaporator at a thermodynamic state (i.e., a pressure and enthalpycondition) in which it is a subcooled liquid or a partially vaporizedtwo-phase fluid of relatively low vapor quality. Thermal energy isdirected into the refrigerant as it travels through the evaporator, sothat the refrigerant exits the evaporator as either a partiallyvaporized two-phase fluid of relatively high vapor quality or asuperheated vapor.

At another point in the system the refrigerant enters a condenser as asuperheated vapor, typically at a higher pressure than the operatingpressure of the evaporator. Thermal energy is rejected from therefrigerant as it travels through the condenser, so that the refrigerantexits the condenser in an at least partially condensed condition. Mostoften the refrigerant exits the condenser as a fully condensed,subcooled liquid.

Some vapor compression systems are reversing heat pump systems, capableof operating in either an air conditioning mode (such as when thetemperature of the uncontrolled ambient environment is greater than thedesired temperature of the controlled environment) or a heating mode(such as when the temperature of the uncontrolled ambient environment isless than the desired temperature of the controlled environment). Such asystem may require heat exchangers that are capable of operating as anevaporator in one mode and as a condenser in an other mode.

In some systems as are described above, the competing requirements of acondensing heat exchanger and an evaporating heat exchanger may resultin difficulties when one heat exchanger needs to operate efficiently inboth modes.

SUMMARY

According to an embodiment of the invention, a heat exchanger isprovided to transfer heat between refrigerant and a flow of air. Theheat exchanger includes a refrigerant flow path that extends between tworefrigerant ports. Three sections of the heat exchanger are arrangedalong the refrigerant flow path. One air flow path extends sequentiallythrough a first section adjacent to one of the refrigerant ports, and asecond section adjacent to the other refrigerant port, while bypassingthe third section. Another air flow path in parallel with the first airflow path extends through only the third section.

In some embodiments, the refrigerant flow path includes at least twopasses through the third section. In some such embodiments therefrigerant flows through those passes in a concurrent-cross flowrelationship with the air.

In some embodiments, the two air flow paths include extended surfacefeatures to promote heat transfer between the air and the refrigerant,and in some such embodiments the spacing density of the extended surfacefeatures is substantially lower in the first section than in the thirdsection. In some such embodiments the first section is substantiallyabsent of extended surface features. In other embodiments the spacingdensity of the extended surface features is substantially lower in thesecond section than in the first section, and in some such embodimentsthe second section is substantially absent of such extended surfacefeatures.

In some embodiments, the refrigerant flow path is defined by flattenedtubes in one or more of the section. In some such embodiments, at leastsome of the flattened tubes are continuous between the first section andat least one pass of the third section. In some such embodiments atleast some of the flattened tubes are continuous between the secondsection and at least one pass of the third section.

According to an embodiment of the invention, a method of removing heatfrom a refrigerant includes separating a flow of air into first andsecond portions. A first quantity of heat is transferred from therefrigerant to the first portion of air, and a second quantity of heatis transferred to the first portion of air after the first quantity ofheat. After the first and second quantities of heat have been removedfrom the refrigerant, a third quantity of heat is transferred from therefrigerant to the second portion of air. The heated first and secondportions of air are then recombined.

In some embodiments, a heat exchanger to transfer heat between arefrigerant and air is provided and comprises a refrigerant flow pathextending between a first refrigerant port and a second refrigerantport; a first section, a second section, and a third section of the heatexchanger arranged sequentially along the refrigerant flow path, thefirst section arranged between the first refrigerant port and the secondsection, the third section arranged between the second refrigerant portand the second section; and first and second parallel arranged air flowpaths extending through the heat exchanger, the first airflow pathextending sequentially through the first section and the third sectionand bypassing the second section, the second airflow path extendingthrough the second section and bypassing the first section and the thirdsection, wherein heat transfer between the refrigerant and air issubstantially inhibited in the first section of the heat exchanger,wherein the second refrigerant port is operatively coupled to anexpansion device to receive cooled refrigerant therefrom when the heatexchanger is operated in an air conditioning mode.

Some embodiments of the present invention provide a method of removingheat from refrigerant, comprising: separating a flow of air into a firstportion and a second portion; transferring a first quantity of heatbetween the refrigerant and the first portion of the air whilesimultaneously inhibiting transfer of heat between the refrigerant andthe second portion of the air; transferring a second quantity of heatbetween the refrigerant and the first portion of the air after the firstquantity of heat has been transferred to the first portion of the air;transferring a third quantity of heat between the refrigerant and thesecond portion of the air after the first and second quantities of heathave been transferred; and recombining the first and second portions toprovide an air flow with a changed temperature.

Some embodiments of the invention provide a method of removing heat froma refrigerant including separating a flow of air into first and secondportions. A first quantity of heat is transferred from the refrigerantto the second portion of air, and a second quantity of heat istransferred from the refrigerant to the first portion of air after thefirst quantity of heat has been transferred to the second portion ofair. After the first and second quantities of heat have been removedfrom the refrigerant, a third quantity of heat is transferred from therefrigerant to the first portion of air. Heat transfer between therefrigerant and the second portion of air is inhibited after the firstquantity of heat has been transferred to the second portion of air. Theheated first and second portions of air are then recombined.

In some embodiments, the refrigerant is de-superheated and condensed bythe removal of the first and second quantities of heat. In some suchembodiments the refrigerant is sub-cooled by the removal of the thirdquantity of heat.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a and 1 b are schematic illustrations of a refrigerant systemoperating in an air conditioning mode and a heating mode, respectively.

FIG. 2 is a pressure vs. enthalpy graph depicting a typical vaporcompression cycle for the system of FIGS. 1 a and 1 b.

FIGS. 3 a and 3 b are diagrammatic illustrations of the fluid flowsthrough a heat exchanger according to some embodiments of the presentinvention.

FIG. 4 is a partial perspective view of a heat exchanger according to anembodiment of the present invention.

FIG. 5 is a partial perspective view of a tube and fin combination foruse in the embodiment of FIG. 3.

FIG. 6 is a plan view of the heat exchanger of FIG. 4.

FIG. 7 is a perspective view of a heat exchanger according to anotherembodiment of the present invention.

DETAILED DESCRIPTION

Before any embodiments of the invention are explained in detail, it isto be understood that the invention is not limited in its application tothe details of construction and the arrangement of components set forthin the following description or illustrated in the following drawings.The invention is capable of other embodiments and of being practiced orof being carried out in various ways. Also, it is to be understood thatthe phraseology and terminology used herein is for the purpose ofdescription and should not be regarded as limiting. The use of“including,” “comprising,” or “having” and variations thereof herein ismeant to encompass the items listed thereafter and equivalents thereofas well as additional items. Unless specified or limited otherwise, theterms “mounted,” “connected,” “supported,” and “coupled” and variationsthereof are used broadly and encompass both direct and indirectmountings, connections, supports, and couplings. Further, “connected”and “coupled” are not restricted to physical or mechanical connectionsor couplings.

A reversible heat pump system 30 capable of operating in either of anair conditioning mode and a heating mode is illustrated schematically inFIGS. 1 a and 1 b, and includes a compressor 17, an expansion device 18,first and second heat exchangers 1 and 19, and a four-way valve 20. Arefrigerant circuit 21 interconnects the various components to define aclosed loop refrigerant circuit through the system.

During operation of the system 30 in an air conditioning mode, asillustrated in FIG. 1 a, the compressor 17 operates to direct a flow ofrefrigerant through the circuit 21 by compressing a superheated vaporrefrigerant from a low pressure state, at point 22 in the system, to ahigh pressure state, at point 23 in the system. The compressed vaporrefrigerant is directed by way of the four-way valve 20 to heatexchanger 19, which operates to reject heat from the refrigerant. Theheat exchanger 19 can be preferably located in an environment that doesnot need to be controlled. For example, the heat exchanger 19 can belocated external to a building so that the rejected heat is dischargedto the ambient environment. Alternatively, the heat exchanger 19 canreject the heat from the refrigerant to another fluid such as, forexample, a liquid coolant, in order to transport the rejected heat toanother location.

With continued reference to FIG. 1 a, the heat exchanger 19 preferablycools and condenses the refrigerant from the superheated vapor state toa sub-cooled liquid state. The expansion device 18 expands therefrigerant from that high pressure sub-cooled liquid state, at point 26in the system, to a low pressure two-phase (vapor-liquid) state, atpoint 27 in the system. The low pressure two-phase refrigerant isdirected into heat exchanger 1, wherein heat is transferred to therefrigerant in order to fully vaporize, and preferably superheat, therefrigerant. The refrigerant exiting the heat exchanger 1 is thendirected by way of the four-way valve 20 back to the inlet of thecompressor 17.

The heat transferred into the refrigerant in the heat exchanger 1 ispreferably transferred from a flow of supply air directed through theheat exchanger 1. The supply air can thereby be cooled and/ordehumidified, and can be supplied to an occupied space in order toprovide climate comfort in that space.

The system 30 can also be operated in a heating mode, illustrated inFIG. 1 b, when conditions dictate that the supply air should be heated.The four-way valve 20 is adjusted so that the compressed refrigerant atpoint 23 is directed by way of the four-way valve 20 to the heatexchanger 1. Heat is removed from the superheated compressed refrigerantin the heat exchanger 1, so that the refrigerant exits the heatexchanger 1 in a sub-cooled liquid state. As will be discussed furtheron in greater detail, in heating mode the refrigerant passes through arefrigerant flow path 10 of heat exchanger 1 in opposite direction ofthe flow through that flow path when operating in air conditioning mode.

With continued reference to FIG. 1 b, the refrigerant is again expandedby the expansion device 18 from the high pressure sub-cooled liquidstate at point 26 to a low pressure two-phase (vapor-liquid) state atpoint 27. The refrigerant is next directed through the heat exchanger19, wherein it receives heat in order to fully vaporize, and preferablysuperheat, the refrigerant. The refrigerant exiting the heat exchanger19 is then directed by way of the four-way valve 20 back to the inlet ofthe compressor 17.

The thermodynamic cycle of the refrigerant passing through the system 30in either the air conditioning mode or the heating mode is illustratedin the pressure-enthalpy diagram of FIG. 2. As discussed previously, therefrigerant is compressed from a relatively low pressure superheatedvapor state at point 22 to a relatively high pressure superheated vaporstate at point 23, is cooled and condensed to a relatively high pressuresub-cooled liquid state at point 26, is expanded to the relatively lowpressure two-phase (vapor-liquid) state at point 27, and is vaporizedand slightly superheated back to the thermodynamic state of point 22.

The rate at which heat is transferred into the refrigerant in eitherheat exchanger 1 (in air conditioning mode) or heat exchanger 19 (inheating mode) can be quantified as the refrigerant mass flow ratemultiplied by the enthalpy change from point 27 to point 22. Likewise,the rate at which heat is transferred from the refrigerant in eitherheat exchanger 19 (in air conditioning mode) or heat exchanger 1 (inheating mode) can be quantified as the refrigerant mass flow ratemultiplied by the enthalpy change from point 23 to point 26. The heatrejected from the refrigerant includes a sensible vapor portion(corresponding to the enthalpy change from point 23 to point 24), alatent portion (corresponding to the enthalpy change from point 24 topoint 25), and a sensible liquid portion (corresponding to the enthalpychange from point 25 to point 26).

In order to improve the heat transfer performance of the heat exchanger1, it can be beneficial for the refrigerant flow path 10 to includemultiple sequential passes through the flow of air passing through theheat exchanger 1. FIGS. 3 a and 3 b illustrate such an arrangement offlow passes for a heat exchanger 1 according to some embodiments of theinvention, with the refrigerant and air flows oriented to be in anoverall counter flow orientation in FIG. 3 a and in an overallconcurrent flow orientation in FIG. 3 b.

In the embodiments of FIGS. 3 a and 3 b, the heat exchanger 1 includesfirst and second refrigerant ports 9 a and 9 b, with the refrigerantflow path 10 extending between those ports. The refrigerant flow path 10includes a flow pass 15 connected to the port 9 a and a flow pass 16connected to the port 9 b. A flow of air 11 is directed in cross flowover each of the passes 15, 16 in sequential fashion. In FIG. 3 a, therefrigerant port 9 b functions as an inlet port and the refrigerant port9 a functions as an outlet port, so that the refrigerant flows firstalong the pass 16 and second along the pass 15. This is generallyreferred to as counter flow operation, as the passes are traversed bythe refrigerant flow in an order that is opposite of the one in whichthey are traversed by the air flow. In contradistinction, in FIG. 3 bthe refrigerant port 9 a functions as an inlet port and the refrigerantport 9 b functions as an outlet port, so that the refrigerant flowsfirst along the pass 15 and second along the pass 16. This is generallyreferred to as concurrent flow operation, as the passes are traversed bythe refrigerant flow in the same order as they are traversed by the airflow.

As previously indicated, the refrigerant system 30 of FIGS. 1 a and 1 bwill have refrigerant flowing along the refrigerant flow path 10 in onedirection when operating in air conditioning mode, and in the oppositedirection when operating in a heating mode. Consequently, the heatexchanger 1 according to the embodiment of FIGS. 3 a and 3 b willexperience counter flow heat transfer between the air and therefrigerant in one such mode, and concurrent flow heat transfer betweenthe air and the refrigerant in the other such mode.

The inventors have found that operating with counter flow heat transferin air conditioning mode provides substantial benefits in minimizing thesize of the heat exchanger 1 for a given amount of heat duty.Consequently, the heat exchanger 1 is then operated with concurrent flowwhen the system 30 is in heating mode. This results in the hightemperature superheated vapor refrigerant (point 23 on thepressure-enthalpy diagram) entering the refrigerant flow path at theport 9 a, and the low temperature sub-cooled liquid refrigerant (point26 on the pressure-enthalpy diagram) exiting the refrigerant flow pathat the port 9 b. Due to the elevated temperature of the refrigerant asit is de-superheated from point 23 to point 24, the portion of the airflow that is in heat transfer with that section of the refrigerant flowpath at the beginning of the pass 15 can be heated to a temperature thatis too high to effectively sub-cool the refrigerant at the end of thepass 16. Insufficient sub-cooling can lead to, among other things,increased refrigerant mass flow and decreased system efficiency.

In order to avoid the undesirable effects of insufficient sub-cooling inheating mode, the heat exchanger 1 is provided with a first section 12,a second section 13, and a third section 14 along the refrigerant flowpath 10. The first section 12 is arranged between the refrigerant port 9a and the second section 13, while the third section 14 is arrangedbetween the refrigerant port 9 b and the second section 16. A portion 11a of the air flow is directed through the section 13 and bypasses thesections 12 and 14, while another portion 11 b of the air flow bypassesthe section 13 and is directed first through the section 12 and secondthrough the section 14. The rate of heat transfer between the portion 11b of the air flow and the refrigerant in the pass 15 is substantiallyinhibited in the section 12, so that the temperature of the air 11 b ismaintained at a sufficiently low temperature to enable desirablesub-cooling of the refrigerant in the section 14.

In some instances, it may instead be preferable to maximize the abilityto transfer heat from the refrigerant to the air flow in heating mode.This can be accomplished by operating with counter flow heat transfer inheating mode (as shown in FIG. 3 a). Consequently, the heat exchanger 1is then operated with concurrent flow (as shown in FIG. 3 b) when thesystem 30 is in air conditioning mode. A similar problem to theaforementioned can then be encountered in air conditioning mode, as theportion of the air flow that is in heat transfer with that section ofthe refrigerant flow path at the beginning of the pass 15 can be cooledto a temperature that is too low to effectively superheat therefrigerant at the end of the pass 16. This problem can also be solvedin the manner described above. Substantially inhibiting the rate of heattransfer between the portion 11 b of the air flow and the refrigerant inthe pass 15 allows for the temperature of the air 11 b to be maintainedat a sufficiently high temperature to enable desirable superheating ofthe refrigerant in the section 14.

The same effects can be alternatively achieved in the heat exchanger 1by instead substantially inhibiting the rate of heat transfer in thesection 14 between the portion 11 b of the air flow and the refrigerantin the pass 16. Insufficient sub-cooling of the refrigerant when theheat exchanger 1 is operated in heating mode with the concurrent flowoperation shown in FIG. 3 b can again be avoided, albeit in a somewhatdifferent manner. In such an alternative embodiment, the transfer ofheat from the refrigerant to the portion of air 11 b serves tode-superheat the refrigerant, so that the temperature of the refrigeranttraveling along the pass 15 is decreased to approximately its saturationtemperature when it enters the section 13 of the heat exchanger 1.Having been thus reduced in temperature, any overheating of that portionof the air 11 a can be avoided, so that the portion 11 a encounters therefrigerant in the pass 16 with a temperature that is sufficiently lowto enable an adequate sub-cooling of the refrigerant within the section13. The refrigerant, having been appropriately sub-cooled, continuesalong the pass 16 through the section 14 to the refrigerant port 9 b.The portion 11 b of the air flow, which has been heated to an elevatedtemperature in section 12, can be at a temperature that is greater thanthe sub-cooled refrigerant as that portion of the air passes through thesection 14. By inhibiting the undesirable transfer of heat from that airto the sub-cooled refrigerant, the refrigerant can be removed from theheat exchanger 1 at a desirable condition.

Turning now to FIGS. 4-6, an especially preferable embodiment of theheat exchanger 1 will be described. As best seen in FIG. 4, the heatexchanger 1 can include first and second tubular manifolds 2 a, 2 b.While not shown in the figures, each of the manifolds 2 can include oneof the refrigerant ports 9. The manifolds 2 are arranged at a common endof the heat exchanger 1, while a return manifold 5 is arranged at theopposite end. The manifolds 2 are provided with slots 6 arranged withregular spacing along their length, and flat tubes 3 are received withinthe slots 6 and extend from the manifolds 2 to the return manifold 5.For clarity, only two flat tubes 3 are shown in FIG. 4, but it should beunderstood that tubes 3 are provided at each of the slots 6. Convolutedfin structures 4 are disposed against, and joined to, the broad sides ofthe flat tubes 3 to provide a plurality of flow channels 28 throughwhich air can pass in cross flow orientation to the flat tubes 3. Again,for clarity, only a single layer of the convoluted fin structures 4 areshown in FIG. 4, but it should be understood that the convoluted finstructures 4 are repeated between each set of adjacent flat tubes 3.

The return manifold 5 can be constructed as shown in co-pending U.S.patent application Ser. No. 13/076,607 with inventors in common to thisapplication, the contents of which are incorporated by reference herein.Alternatively the return manifold can be constructed in other ways, suchas with an additional pair of tubular manifolds with a fluid connectiontherebetween. In some embodiments the flat tubes 3 can be long flattubes with a centrally located bend separating two straight lengths,each straight length being joined to one of the two manifolds 2.

As best seen in FIG. 5, the flat tubes 3 can be provided with internalwebs 7 to provide a plurality of micro-channels 8 within each of theflat tubes 3. In some embodiments the heat exchanger 1 can include roundtubes in place of flat tubes, and/or plate fins in place of theconvoluted fins 4.

Heat transfer between a flow of air passing over the flat tubes 3 and aflow of refrigerant passing through the internal channels of the flattubes 3 is inhibited in a region 12 immediately adjacent to the manifold2 a by the elimination of the convoluted fin structures 4. The pluralityof flow channels 28 created by the convoluted fin structures 4 along theremaining length of the flat tubes 3 connected to the manifold 2 a serveto maintain separation between that portion of the air flow 11 passingthrough the section 13 and that portion of the air flow 11 passingthrough the section 12. The portion of the air flow passing through thesection 12 is maintained at a relatively unchanged temperature. Aspreviously described, in some embodiments the heat transfer can insteadbe inhibited in the section 14, rather than in the section 12, inessentially similar fashion.

In some embodiments, the manifold 2 a includes a refrigerant port 9 toreceive a flow of refrigerant from a compressor 17 in heating mode. Afirst quantity of heat is removed from the refrigerant as it flowsthrough the section 13 along the first pass 15 to the return manifold 5.A second quantity of heat is removed from the refrigerant as it flowsfrom the return manifold 5 through the section 13 along the second pass16. The refrigerant next passes through the section 14 to the manifold 2b, in heat transfer relationship with the portion of the air flow thatpassed through the section 12.

As a result of the transfer of the first quantity of heat to the portionof air in the section 13, that portion of the air may be heated to atemperature at which it can condense the refrigerant, but cannoteffectively sub-cool it. Consequently, the sum of the first and secondquantities of heat corresponds to an enthalpy change of the refrigerantfrom the point 23 on the pressure-enthalpy diagram to the point 25, sothat the refrigerant exits the section 13 as a saturated liquid. Becausethe air passing through the section 14 has been maintained at asubstantially constant temperature, it is cool enough to remove theremaining amount of heat necessary to reduce the enthalpy of therefrigerant from that of point 25 to that of point 26, so that therefrigerant is delivered to the manifold 2 b as a sub-cooled liquid.

In some embodiments, the manifold 2 a includes a refrigerant port 9 toreceive a flow of refrigerant from an expansion device 18 in airconditioning mode. A first quantity of heat is transferred to therefrigerant as it flows through the section 13 along the first pass 15to the return manifold 5. A second quantity of heat is transferred tothe refrigerant as it flows from the return manifold 5 through thesection 13 along the second pass 16. The refrigerant next passes throughthe section 14 to the manifold 2 b, in heat transfer relationship withthe portion of the air flow that passed through the section 12.

As a result of the transfer of the first quantity of heat from theportion of air in the section 13, that portion of the air may be cooledto a temperature at which it can vaporize the refrigerant, but cannoteffectively superheat it. Because the air passing through the section 14has been maintained at a substantially constant temperature, it is warmenough to provide the remaining amount of heat necessary to increase theenthalpy of the refrigerant to that of the point 22 on thepressure-enthalpy diagram, so that the refrigerant is delivered to themanifold 2 b as a superheated vapor. In some alternative embodiments ofthe heat exchanger 1, a fin structure having a substantially decreasedfin density can be provided in the section 12 in place of the un-finnedregion. In some alternative embodiments a single convoluted finstructure can extend across both rows of the flat tubes 3 in the section13. In some embodiments the convoluted fin structure 4 in the first pass15 can have a different fin density than the convoluted fin structure 4in the second pass 16.

An alternative heat exchanger embodiment 1′ is shown in FIG. 7. In theembodiment 1′, the tubular manifold 2 a is relocated to provide aseparation between the section 12 and the section 13 of the heatexchanger.

Various alternatives to the certain features and elements of the presentinvention are described with reference to specific embodiments of thepresent invention. With the exception of features, elements, and mannersof operation that are mutually exclusive of or are inconsistent witheach embodiment described above, it should be noted that the alternativefeatures, elements, and manners of operation described with reference toone particular embodiment are applicable to the other embodiments.

The embodiments described above and illustrated in the figures arepresented by way of example only and are not intended as a limitationupon the concepts and principles of the present invention. As such, itwill be appreciated by one having ordinary skill in the art that variouschanges in the elements and their configuration and arrangement arepossible without departing from the spirit and scope of the presentinvention.

We claim:
 1. A heat exchanger to transfer heat between a refrigerant andair, comprising: a refrigerant flow path extending between a firstrefrigerant port and a second refrigerant port; a first section, asecond section, and a third section of the heat exchanger arrangedsequentially along the refrigerant flow path, the first section arrangedbetween the first refrigerant port and the second section, the thirdsection arranged between the second refrigerant port and the secondsection; and first and second parallel arranged air flow paths extendingthrough the heat exchanger, the first airflow path extendingsequentially through the first section and the third section andbypassing the second section, the second airflow path extending throughthe second section and bypassing the first section and the thirdsection, wherein heat transfer between the refrigerant and air issubstantially inhibited in the third section of the heat exchanger. 2.The heat exchanger of claim 1, wherein the refrigerant flow pathcomprises at least two passes through the second section, refrigerantflowing through said at least two passes in a concurrent flow heattransfer relationship to the air when the heat exchanger is operated ina heat pump mode.
 3. The heat exchanger of claim 1 further comprising aplurality of extended surface features arranged along the first andsecond air flow paths to promote heat transfer between the air and therefrigerant.
 4. The heat exchanger of claim 3, wherein the spacingdensity of the extended surface features in the third section issubstantially lower than the spacing density of the extended surfacefeatures in the second and first sections.
 5. The heat exchanger ofclaim 3, wherein the third section is substantially absent of saidextended surface features.
 6. The heat exchanger of claim 1, furthercomprising a plurality of flattened tubes to define the refrigerant flowpath in one or more of the first, second, and third sections of the heatexchanger.
 7. The heat exchanger of claim 6, wherein the refrigerantflow path comprises at least two passes through the second section, theplurality of flattened tubes includes a first plurality of flattenedtubes defining one of the at least two passes, and the plurality offlattened tubes includes a second plurality of flattened tubes defininganother one of the at least two passes.
 8. The heat exchanger of claim7, wherein the first plurality of flattened tubes further defines therefrigerant flow path in the third section of the heat exchanger.
 9. Theheat exchanger of claim 8, wherein the second plurality of flattenedtubes further defines the refrigerant flow path in the first section ofthe heat exchanger.
 10. A method of removing heat from refrigerant,comprising: separating a flow of air into a first portion and a secondportion; transferring a first quantity of heat from the refrigerant tothe second portion of the air; transferring a second quantity of heatfrom the refrigerant to the first portion of the air after the firstquantity of heat has been transferred to the second portion of the air;transferring a third quantity of heat from the refrigerant to the firstportion of the air after the first and second quantities of heat havebeen transferred; inhibiting the further transfer of heat between therefrigerant and the second portion of air after the first quantity ofheat has been transferred to the second portion of air; and recombiningthe first and second portions to provide an air flow with a changedtemperature.
 11. The method of claim 10, wherein transferring the firstquantity of heat desuperheats the refrigerant.
 12. The method of claim10, wherein transferring the second and third quantity of heat condensesand subcools the refrigerant.
 13. The method of claim 10, furthercomprising passing the air and the refrigerant through a heat exchangerto transfer the first, second, and third quantities of heat.
 14. Themethod of claim 13, further comprising: passing the refrigerant througha section of the heat exchanger after transferring the first, second andthird quantities of heat from the refrigerant; and passing the secondportion of the air through said section of the heat exchanger aftertransferring the first quantity of heat to the second portion of theair, wherein the temperature of the second portion of the air issubstantially unchanged as it passes through said section of the heatexchanger.
 15. The method of claim 10, further comprising movingrefrigerant in concurrent flow with respect to the first and secondportions of air.